1. Field
Example embodiments relate to nonvolatile semiconductor memory devices.
2. Description of the Related Art
Semiconductor memories are usually considered to be the most vital microelectronic component of digital logic system design, such as computers and microprocessor-based applications ranging from satellites to consumer electronics. Therefore, advances in the fabrication of semiconductor memories including process enhancements and technology developments through the scaling for higher densities and faster speeds help establish performance standards for other digital logic families.
Semiconductor memory devices may be characterized as volatile random access memories (RAMs), or nonvolatile memory devices. In RAMs, the logic information is stored either by setting up the logic state of a bistable flip-flop such as in a static random access memory (SRAM), or through the charging of a capacitor as in a dynamic random access memory (DRAM). In either case, the data is stored and can be read out as long as the power is applied, and are lost when the power is turned off; hence, they are called volatile memories.
Nonvolatile memories, such as Mask Read-Only Memory (MROM), Programmable Read-Only Memory (PROM), Erasable Programmable Read-Only Memory (EPROM), and Electrically Erasable Programmable Read-Only Memory (EEPROM), are capable of storing the data, even with the power turned off. The nonvolatile memory data storage mode may be permanent or reprogrammable, depending upon the fabrication technology used. Nonvolatile memories are used for program and microcode storage in a wide variety of applications in the computer, avionics, telecommunications, and consumer electronics industries. A combination of single-chip volatile as well as nonvolatile memory storage modes is also available in devices such as nonvolatile SRAM (nvSRAM) for use in systems that require fast, programmable nonvolatile memory. In addition, dozens of special memory architectures have evolved which contain some additional logic circuitry to optimize their performance for application-specific tasks.
In nonvolatile memories, however, MROM, PROM, and EPROM are not free to be erased and written to by a system itself, so that it is not easy for general users to update stored contents. On the other hand, EEPROM is capable of being electrically erased or written. Application of an EEPROM is widened to an auxiliary memory or to system programming where continuous updates are needed (flash EEPROM).